In the UK it is mandatory that you wear one unless your are a Seikh. They do not have to wear them because of their turbans, but how a piece of cloth can stop your head being caved in when it his the tarmac is another question !!!.
I think it's absolutley crazy not to wear one.
I once hired a moped on holiday when I was much younger and more stupid and rode round all day without a helmet - I would never do that now.
It would only take the slightest skid to make you come off, and BAM thats it game over.
My g/f's dad was out on his bike on an advanced rider course with a Police riding instructor, when going round a bend his back wheel slipped on some loose gravel and he came off his bike. His helmet was craked open and scuffed, but his head was fine - now without the helmet he would not be here today. Accidents can happen even to the most cautious of people at any time - without a helmet your D-E-A-D.
"She Rolls, 45 knots, 90, 135, nose comes up to 20 degrees, she's airborne - She flies, Concorde Flies"